Pounds and ounces are not the unit of mass but a unit of Weight. Weight is different from a mass because a mass becomes weights if it is subjected to a gravitational force or acceleration. The weight of objects at sea level is different when the object is placed at a higher elevation or when the object is placed in moon where there is no gravity there.

Mass is a property of matter, and its units are usually measured in grams (g) or kilograms (kg) worldwide and additionally ounces (oz) and pounds (lbs) in the US. On the small scale, atomic mass, or the mass of elemental particles (protons, neutrons, electrons, and whole atoms) are measured relative to an atom of carbon-12, which is set to have a mass of 12.0000. Mass should not be confused with weight, which a measurement of how heavy matter is when subject to the force of gravity, so:

Mass is "the amount of stuff" in an object. That is, the amount of atoms and molecules present. Mass in science, is measured in the metric system using units such as kilogram (about 2.2 pounds)nwhich are a thousand grams, grams, centigrams (hundredths of grams), and milligrams (thousandths of grams). Mass is different from weight, Weight is a force. It depends on gravity pulling us toward the center of our planet (or another planet if we ever get there or the moon). If we weighed ourselves on our bathroom scale and noted our weight and then took our scale with us to the moon, we would find that our weight was only one sixth as much there because the force of gravity is much smaller. However, because when we find mass, we use a procedure that compares our object with known mass, our mass would not change regardless of the amount of pull of gravity. This explains why astronauts did not change their size when they were on the moon even though their weight decreased considerably. Well, OK, that's not exactly true. Their size changed just a little. They were temporarily a tiny bit taller because gravity was not pulling them down, compressing their spines a little bit. But that has nothing to do with the definition of mass.

A unit of mass is a measurement of mass. It is actually a measurement of how much matter there is.The closest concept is weight. Let's understand the difference between weight and mass. A 50 lb. dumbell is twice the mass of a 25lb. weight.If these dumbells are on the surface of the moon,they will weigh less because of less gravitational pull.The change of weight might be seen by being able to lift the weights with two fingers as though it was styrofoam.So weight is variable in relation to gravitational pull.However the ''amount of matter'' or ''mass'' did not change when it was on the moon.

One way of measuring mass is used in particle physics. The ''atomic weight''( despite it's name) is one scale used to measure the mass of different elements.Hydrogen has an atomic weight of 1. It contains one proton.Helium has an atomic weight of 4 and its nucleus has 2 protons + 2 neutrons.

The property a body has of resisting any change in its state of rest or uniform motion is called inertia. The inertia os a body is related to what we think of as the amount of matter it contains. A quantitative measure of inertia is mass: The more mass a body has, the less its acceleration when a given net force acts on it. The SI unit of mass is the kilogram (kg).

The metric unit of mass is 1 kilogram, as represented by the International Prototype Kilogram stored in Paris.

The gravitational pull that the Earth exercises towards a 1-kilogram mass is 1kg * 1 G = (roughly) 9.81 kgm/s2 = 9.81 Newton. Note that the Earth's gravitational pull varies by 1% depending on where you are on the Earth.

An earlier answer suggested that the Moon has no gravity. Not true. The gravitational pull of the Moon is roughly 1/6 of Earth's gravity. The Standard kilogram would therefore weigh some 0.166 kilograms in the Moon.

The SI unit of mass is the kilogram (kg). As mass is difficult to measure directly, usually balances or scales are used to measure the weight of an object, and the weight is used to calculate the object's mass.

Many may find it difficult or confusing to differentiate mass from weight. Mass is how much matter an object contains while weight is the force of gravity acting upon that object. Therefore, the SI unit of weight is N (Newton) while that of mass is kg (kilogram).

The amount of constituent particles that a matter contains is said to be its mass. Its very different in respect to weight. It is common misconception that mass and weight are almost same but they are very different infact. Coming to the point the SI units of mass is kilogram(Kg).

Mass is the total amount of matter in an object. Basically it is the total weight of an object in terms of kilogram for large objects, grams for small objects and for very small objects milligrams is used.

According to different system of unit are used in the different parts of the world. These systems are mainly the methods of defining the three fundamental units. The most widely used system of units is CGS or French or metric system.

In 1960, at Eleventh General Conference on Weights and Measures held at Paris, an international system of units is adopted to have a consitent system of units and also to simplify the communications amon the scientists. This system of units is known as Le Systeme International d' Unites abbreviated as SI units.

C.G.S. or French or Metric System: This system was originated in France but now it is used in the scientific measurements and also is widely used throughout the world. In this system, the unit of length is centimetre, the unit of mass in gram and the units of time is second. From the initial letters of the words centimetre, gram and second this system is called the C.G.S. system.

S.I. units: In this system in addition to units of length, mass and time, units of electric current, temperature, luminous intensity and quantity of substance i.e., seven basic units in all are adopted. These are-